Solutions of polyaluminoxane compositions are generally condensation products prepared by the partial hydrolysis of an organic aluminum compound, and are known to be useful as cocatalyst components for efficiently activating transition metal compounds serving as primary catalysts in the manufacturing of olefin polymers. Polymethylaluminoxane compositions employing trimethylaluminum as starting material organic aluminum compounds are widely known to exhibit particularly good cocatalyst performance. These compositions are normally handled in the form of solutions obtained by dissolving them in aromatic hydrocarbon solvents such as toluene.
Polymethylaluminoxane compositions exhibit good cocatalyst performance. However, because they are normally handled in the form of solutions obtained by dissolving both a primary catalyst such as a metallocene compound and a polymethylaluminoxane composition in a solvent, it is impossible to control the morphology of the polymer that is produced. Thus, not only does handling of the polymer become difficult, there is a problem in the form of a strong tendency for occurrence of fouling due to adhesion of the polymer to the polymerization reactor or the like.
The method of applying a support-type solid polymethylaluminoxane composition—in which a polymethylaluminoxane composition is supported on a solid inorganic support such as silica, alumina, or magnesium oxide—to suspension polymerization or gas phase polymerization has been proposed to solve these problems. Among the solid inorganic supports, silica in which the quantity of surface hydroxyl groups is controlled is the most widely employed as a support, with a number of examples of extension to the industrial level. (For example, see Patent Reference 6 and the like.)
The above silica support tends to remain within the polymer and is known to compromise the performance of the polymer. Further, solid polymethylaluminoxane compositions in which such supports are employed are known to exhibit a large drop in polymerization activity relative to the activity in homogeneous polymerization. Accordingly, to solve the above problem, it would be desirable to develop a solid polymethylaluminoxane composition with high activity rivaling that of homogeneous polymerization while retaining the merit of being solid of polymethylaluminoxane compositions as a cocatalyst.
Attempts have been made to obtain solid polymethylaluminoxane compositions without the use of solid supports such as those set forth above. Generally, when attempting to prepare a solid polymethylaluminoxane composition, the method of reacting some sort of additive with a solution of polymethylaluminoxane composition is adopted (see Patent References 1 to 5 and the like). However, in this method, the recovery ratio of solid material based on aluminum is not high.
In some cases, the addition of a third component to obtain a solid polymethylaluminoxane composition creates problems such as toxicity, depending on the use of the polymer. Such addition is thus better avoided. Due to such considerations, there have been proposed methods of obtaining solid polymethylaluminoxane compositions by adding just a solvent of non-solubility or poor-solubility to a solution of a polymethylaluminoxane composition. Patent References 1 and 5 propose methods of adding n-hexane or n-decane to a toluene solution of a polymethylaluminoxane composition to cause the solid polymethylaluminoxane composition to precipitate, and then increasing the amount of precipitation by removal of the solvent using a vacuum pump.
Although the yield and average particle diameter of the solid polymethylaluminoxane composition that precipitates are recorded in the methods set forth in Patent References 1 and 5, other than these, there is no mention of morphologies.
When combining a solid polymethylaluminoxane composition with a transition metal compound to prepare an olefin polymer, the properties of the olefin polymer being prepared are greatly influenced by the morphology of the solid polymethylaluminoxane composition. Generally, the solid polymethylaluminoxane composition with fine particles and a relatively uniform particle diameter is desirable because the olefin polymer that is prepared will have more uniform particles. The particle diameter of the solid polymethylaluminoxane composition described in the embodiments of Patent Reference 1 is 210 to 350 μm, and the particle diameter of the solid polymethylaluminoxane composition described in Patent Reference 5 is 28 to 47 μm. That is, in these patent references, there is no description of, and it is unclear, what must be done to prepare a solid polymethylaluminoxane composition with a small particle diameter of 30 μm or less. It is also unclear to what degree the particle diameter of the solid polymethylaluminoxane composition described in the embodiments is uniform.
There is also a problem in that a solid polymethylaluminoxane composition will generally have lower polymerization activity than a solution of polymethylaluminoxane composition. The yield of a solid polymethylaluminoxane composition is also low and the cost is high because the entire quantity of the polymethylaluminoxane contained in a solution of polymethylaluminoxane composition is not recovered as solid polymethylaluminoxane composition. When obtaining a solid polymethylaluminoxane composition such as by the methods described in Patent References 1 and 5, the method of removing a large amount of solvent from the solution of polymethylaluminoxane composition with a vacuum pump presents no problem and can be implemented at the lab level, but when considering implementation on an commercial scale, there is a possibility that polymethylaluminoxane or trimethylaluminum that does not solidify will scatter toward the vacuum pump. This is not only dangerous, but presents numerous problems from the perspective of production efficiency, and is not a practical way to conduct operations.
The present inventors recently discovered a solid polymethylaluminoxane composition with relatively fine particles with a median diameter based on volume falling within a range of 5 to 50 μm and with relatively uniform particle diameter, and a method for efficiently preparing the same (Patent Reference 7) without using a solid support such as SiO2. The solid polymethylaluminoxane composition obtained in Patent Reference 7 is characterized not only by relatively fine particles of a particle diameter that is relatively uniform, but also by the fact that the polymerization activity is extremely high in the course of preparing an olefin polymer. Depending on the conditions, activity rivaling that of solution polymethylaluminoxane compositions is sometimes exhibited.
In this process, it is possible to provide a method such that the yield of polymethylaluminoxane composition in the process of preparing a solid polymethylaluminoxane composition from a solution of a polymethylaluminoxane composition is high and the removal of solvent by vacuum pump from the solution of polymethylaluminoxane composition is unnecessary.